Use of photosensing systems in industry and other areas has become prevalent in recent years. Such systems include a photoelectric transmitter that projects a source of light (preferably a source of modulated light) and a photoelectric receiver for detecting the projected light. A photoelectric transmitter and photoelectric receiver pair may be mounted in the same housing or they may be mounted in separate housings that are spaced apart when in use. When the transmitter/receiver pair are mounted in the same housing, a diffuse reflecting, or retroreflective surface is used to redirect the source light from the transmitter to the receiver. When the transmitter and receiver pair are spaced apart, the source light usually impinges directly on the receiver. A step change, through the threshold detection level, in the amount of light impinging on the receiver creates control signal information used to control or communicate with various types of equipment. The change may be the interruption of a light beam caused by a passing object or the receipt of a light beam normally blocked by the presence of an object.
One requirement of photosensing systems is the need to properly align an associated transmitter and receiver pair when they are spaced apart or to properly align a retroreflective surface with the transmitter/receiver pair when the transmitter/receiver pair are mounted in the same housing. Proper alignment provides increased sensor immunity to dust and other environmental optical contaminants that degrade performance of photosensing systems over time.
In the past, efforts to align transmitter/receiver pairs have been hindered by the lack of an accurate apparatus for indicating precise alignment. Among other apparatus, the prior art has used multiple LEDs in the form of a bar graph attached to the sensors to indicate photosensor alignment. One disadvantage of such apparatus is that alignment indication is limited to the discrete steps provided by the number of LEDs used. Other disadvantages include the comparatively high expense of such apparatus and the bulk of such apparatus which makes their use impractical of miniature type sensors. Blinking LED systems, such as that disclosed in U.S. Pat. No. 4,356,393, wherein the LED blinking rate indicates the degree of alignment, have also been used by the prior art. Unfortunately, variations in LED blinking rates are difficult to visually perceive. Also, LED blinking is easily confused with sensor detection of passing objects. Additionally, prior art devices and apparatus have been plagued by the inability to respond to the broad dynamic range of light signals required as a result of their linear operating characteristics which limit perceptible indication over the broad dynamic range of signal strength. Linear indicating systems cannot show the 50:1, or higher, dynamic range of signal strength reception available with photosensing systems. It is relatively common to align a photoelectric sensor on the fringes of its sensing zone and not know that optimum sensor alignment for immunity to environmental contamination has not been achieved.
Therefore, as can be readily appreciated from the foregoing discussion, there is a need for an apparatus to assist in the accurate alignment of photosensing systems. The present invention is directed to providing such an apparatus, namely a low cost signal strength indicator suitable for use with miniature type photosensing systems that provides a visual indication of the degree of photosensor alignment and which also has the capability to respond to a broad dynamic range of inputs.